1. Field of the Invention
The invention relates to a device for imaging the interior of a turbid medium, which device includes a holder which has an open side bounded by an edge portion, encloses a measuring volume and is arranged to receive a matching liquid and the turbid medium, a light source for irradiating the turbid medium and the matching liquid, a photodetector for measuring a part of the light transported through the turbid medium and the matching liquid, and a control unit for reconstructing an image of the interior of the turbid medium on the basis of the measured intensities.
2. Description of Related Art
In the context of the present patent application the term light is to be understood to mean electromagnetic radiation of a wavelength in the range of from 400 to 1400 nm. Furthermore, a turbid medium is to be understood to mean a substance consisting of a material having a high light scattering coefficient. Examples in this respect are an Intralipid solution or biological tissue. Furthermore, an attenuation coefficient is to be understood to mean the inverse diffuse absorption distance xcexa which is given by xcexa=3xcexcaxcexcxe2x80x2s, where xcexcxe2x80x2s represents the reduced scattering coefficient and xcexca represents the absorption coefficient.
A device of the kind set forth is known from the patent application WO 99/03394. Such a device could be used for in vivo examinations for detecting the presence of any tumors in breast tissue of a human or animal female. To this end, the turbid medium, in this case being, for example, a part of the breast of the female to be examined, is immersed in the matching liquid in the holder. The holder also includes a first number of light sources and a second number of photodetectors which are distributed across the wall of the holder. The matching liquid provides optical coupling between the part of the breast to be imaged and the light sources and the photodetectors, respectively, in the holder. Furthermore, the optical parameters of the matching liquid, such as the reduced scattering coefficient xcexcxe2x80x2s and the absorption coefficient xcexca, are approximately equal to those of the part of the breast to be imaged. Furthermore, the matching liquid prevents optical short-circuiting between the light sources and the photodetectors. In the context of the present patent application an optical short-circuit is to be understood to mean a light path between one of the light sources and one of the photodetectors in the holder which does not extend through the part of the breast in the holder. Finally, the matching liquid counteracts boundary effects in the reconstructed image; such effects are caused by the difference in optical contrast between the interior of the breast tissue and the remaining space in the holder. In order to measure the intensities, alternately one of the light sources irradiates the part of the breast to be imaged and the photodetectors measure a part of the light transported through the part of the breast to be imaged. These measurements are repeated until the part to be imaged has been irradiated by all light sources present in the holder. The results of the measurements are stored in a memory of the control unit. The control unit subsequently reconstructs the image of the interior of the part of the breast to be imaged from the measured intensity measurements.
It is a drawback of the known device that artefacts, for example contrast spots, are liable to occur in the reconstructed image of the part of the breast to be imaged. As a result, it is impossible to observe contrast differences which are possibly caused by the presence of a tumor which is small in comparison with such a contrast spot. The image may also provide a false indication of a tumor in the part of the breast where no tumor is found after other diagnostic examinations.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant""s invention of the invention subsequently claimed.
It is an object of the device according to the invention to counteract the occurrence of said artefacts in the reconstructed image. To this end, the device according to the invention is characterized in that it is provided with an elastically deformable sealing ring which is provided on the edge portion of the holder. The invention is based on the recognition of the fact that one possible cause of said artefacts resides in the fact that the part of the turbid medium to be imaged often is not completely immersed in the matching liquid. Air is trapped in locations in the holder which do not contain matching liquid and in which the turbid medium does not directly contact the holder. Consequently, the desired optical coupling is not realized between the light sources and the part of the turbid medium to be imaged and/or between the part of the turbid medium to be imaged and the photodetectors. For example, when the turbid medium is a breast of a female to be examined, it occurs that the dimensions of the part of the breast to be imaged are smaller than the part to be imaged of an average breast for which the fixed holder has been designed, so that a large free space is liable to occur to the left or to the right of the breast in the fixed holder. This also gives rise to a large boundary with air present outside the holder, near the upper openings of the photodetectors. A further possibility is that the positioning of the breast in the holder removes matching liquid from the holder so that an air gap is formed or an air bubble is trapped. Furthermore, it may be that during the measurement a small amount of matching liquid is transported out of the holder due to motion of the body to be examined. By providing the elastically deformable sealing ring on the edge portion of the holder, the holder can be made to adjoin exactly the surface around the breast of the body to be examined. The elastically deformable sealing ring thus counteracts the escape of the liquid from the holder. It is a further advantage of the sealing ring that the level of the matching liquid can be increased so that the distance between the photodetectors near the edge portion in the holder and the surface of the matching liquid is increased. It is a further advantage that respiratory effects are precluded. As a result of the described advantages, said artefacts are reduced and the reproducibility of the reconstructed images is enhanced so that they are better suited for diagnostic purposes. That is, as the artefacts are reduced, the diagnostic quality is improved.
A special embodiment of the device according to the invention is characterized in that the sealing ring contains polyurethane (PUR) of a density in a range of from 0.01 to 0.4 kg/l. PUR is a material allowing economical manufacture of the sealing ring in large numbers. For this density the PUR has properties such that the sealing ring can be almost completely compressed under the weight of the body to be examined.
A further embodiment of the device according to the invention is characterized in that the sealing ring contains an internal chamber which extends along at least a part of the circumference of the sealing ring.
A further embodiment of the device according to the invention is characterized in that the internal chamber contains a gas. The elasticity of the sealing ring can be adapted to the weight of the body to be examined by filling the internal chamber with a gas or a mixture of gases, for example air.
A further embodiment of the device according to the invention is characterized in that the internal chamber is connected to a rigid pressure vessel. The sealing ring is connected to the rigid pressure vessel, for example via a tube. The rigid pressure vessel acts as a buffer volume, so that the pressure in the chamber of the sealing ring remains approximately constant even when the sealing ring is compressed. The pressure in the pressure vessel is then, for example 100 hPa higher than the atmospheric pressure.
A further embodiment of the device according to the invention is characterized in that the sealing ring is made of a material containing latex or silicon.
Another embodiment of the device according to the invention is characterized in that a diffuse reflector is provided on a side of the sealing ring which faces the measuring volume of the holder. A high diffusion reflection is advantageous in counteracting artefacts in the reconstructed image. The diffuse reflector can be provided, for example by coating the sealing ring with a lacquer on a titanium dioxide base.
A further embodiment of the device according to the invention is characterized in that the sealing ring is provided with an adapter ring for attaching the sealing ring to the edge portion of the holder.
A further embodiment yet of the device according to the invention is characterized in that the device includes a reservoir for storing matching liquid and a pump which is connected between the reservoir and the holder in order to adjust the level of the matching liquid to be received by the holder. An escaped quantity of liquid can be replenished by supplying matching liquid by means of the pump. The quantity of matching liquid pumped into the holder amounts to, for example 100 ml/minute.
The invention also relates to a sealing ring for use in said device.